The invention relates to a power supply circuit for converting an input d.c. voltage into a first output d.c. voltage for application to a first load and into a second output d.c. voltage for application to a second load, the power supply circuit comprising a series arrangement of an inductor and a controllable switch, which series arrangement is connected to a terminal for the input voltage, said switch being, in operation, alternately in the conductive and in the non-conductive states, the first output voltage being derived from a square-wave voltage present across the inductor, the conduction periods of the switch being controlled by means of a control circuit connected to a control electrode of the switch for obtaining for the first output voltage a value which is substantially independent of variations of the input voltage and of the first load, the power supply circuit further comprising a tuned rectifier coupled to the inductor for producing the second output voltage across a smoothing capacitor, said rectifier being such that a half cycle of sinusoidal current flowing therethrough during a period commencing when the switch commences to be conductive does not exceed the conduction time of the switch.
Such a power supply circuit is disclosed in British Patent Application No. 2,056,794 A. In this known circuit the conduction period of the switch is controlled in dependence on variations of the first load and is substantially not influenced by the second load due to the fact that no current flows any longer through the rectifier when the switch is turned off. The rectifier is a peak rectifier which rectifies the peaks of the voltage present across a winding coupled with the inductor for producing the second output voltage. Since the amplitude of these peaks depends on on the first output voltage and therefore on the duty cycle of the switch, it will be clear that the first load has an influence on the second load, which may be undesirable.